The present invention generally relates to fluid dispensing systems and, more particularly, relates to a valve for controlling the pressure and flow in a recirculation loop of an intermittent fluid dispensing system.
The use of variable restrictors in intermittent fluid dispensing systems is generally known for the purpose of controlling the flow of fluid in a recirculation loop of the system when the dispenser is shut off between dispensing cycles. One such variable restrictor is disclosed, for example, in U.S. Pat. Nos. 4,200,207 and 4,850,514 both being assigned to the assignee of the present invention. The variable restrictor disclosed in each of these patents generally controls either the flow rate or the pressure of polymeric material in the recirculation loop by adjusting the size of an aperture within the variable restrictor. For example, with a positive metering pump which maintains a constant flow rate, the variable restrictor will affect the pressure drop across the recirculation loop. However, in a system which maintains a constant pressure the variable restrictor will affect the flow rate of fluid in the recirculation loop.
While the above-mentioned variable restrictor does allow adjustment of the flow rate or the pressure in the recirculation loop within a predetermined range, the adjustment provided by the variable restrictor tends to be quite "nonlinear" and "sensitive" in nature. The nonlinear, sensitive relationship between adjustment of the restrictor and change in flow rate or pressure causes precise control of flow rate and pressure to be rather difficult. This is because, for example, the first turn of the valve stem of the restrictor will not produce the same change in flow rate or pressure as the next turn of the valve stem. Sensitivity of such past variable restrictors is high in that, for example, a slight adjustment might have a relatively large effect on flow rate or pressure. Thus, depending on the specific flow rate and viscosity of polymeric material in the system containing the variable restrictor, as mentioned above, adjustment of the restrictor will generally be nonlinear and the pressure in the system will also sometimes rise and/or fall erratically. Flow and pressure adjustments made with past variable restrictors often involves a time consuming trial-and-error procedure due to high sensitivity of the adjustments. As a result, there is a need in the art for a variable restrictor which is easily adjustable in a more linear manner and which has less adjustment sensitivity and therefore a wider effective adjustment range than variable restrictors of the past.
Another problem associated with prior variable restrictors such as the one mentioned above is that the use of a variable size aperture within the restrictor presents difficulties when the aperture size is severely reduced or, in other words, during extremely low flow rate and high pressure conditions. When the aperture is severely reduced in size, it can easily become plugged or blocked by char or other impurities or debris within the liquid flowing through the recirculation loop of the system. Thus, it would be advantageous to provide a variable restrictor which does not rely on a variable size aperture to adjust pressure and flow.
It has therefore been one object of the present invention to prevent impurities from impeding flow through a variable restrictor.
It has been another object of the invention to establish a more linear relationship between adjustment of the variable restrictor and the corresponding change in pressure within a fluid system and, more specifically, within the recirculation loop of a dispensing system fluid.